What is the cage of reality, and in what ways does it apply to Notes of a Native Son or any of Baldwin's works?

We take our shape, it is true, within and against that cage of reality bequeathed us at our birth; and yet it is precisely through our dependence on this reality that we are most endlessly betrayed.

The quotation above (from the book Notes of a Native Son) represents one of countless attempts in James Baldwin’s writing to capture in a few words the essential challenge of coming to terms with one’s identity while living in a society that reinforces social groups and divisions. A great deal of Baldwin’s work, especially his nonfiction, is concerned with the difficulties facing African Americans and the intricacies of relations between blacks and whites in the United States. Racial differences and racial prejudices constitute the given reality of American life—and their effects are undoubtedly very real. However, these social conditions can offer very little to individuals, of whatever hue or social status, in discovering who they are. In truth they offer only a trap, a cage.
Many times across Baldwin’s writing he warns—explicitly in essays such as “Notes of a Native Son” and by example in fictional works such as Another Country—of the perils facing African Americans (and others) who inwardly accept even a shred of what other people think of them. To arrive at a genuine discovery of who one truly is as a human being, Baldwin believes, requires the shedding or transcending of all the labels, limits, and notions seemingly imposed by society’s categories. The cage is real enough, but it is possible—and necessary—to escape from that cage in order to view the much vaster reality outside oppressive social structures such as racism.
Baldwin is referring to this same process of liberation when he writes the following in the “Autobiographical Notes” that begin Notes of a Native Son: “I have not written about being a Negro at such length because I expect that to be my only subject, but only because it was the gate I had to unlock before I could hope to write about anything else.”